https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61400
--- Comment #1 from Yichao Yu <yyc1992 at gmail dot com> --- Additional info. It seems that G++ only raise this error when all of the following are met: 1. Both of the functions (the one that uses suffix return type and the one used in the argument of decltype) must be non-static template member functions. 2. The function used in decltype have to be explicitly specialized using at least one of the template parameter from the other function. (i.e. there won't be any error if I replace `decltype(a<i>())` with `decltype(a<0>())` or if the template argument can be deduced from the function arguments) and the type of the template argument doesn't seem to matter.
